This invention relates to a vacuum pump of a type capable of producing high vacuums in closed chambers while avoiding hydrocarbon backstreaming. The disclosed invention incorporates principles of turbomolecular pumps; yet it is a unitary device not requiring a separate forepump.
While turbomolecular pumps are well known to the pumping art, their application has been limited in spite of their ability to produce high vacuum because of a number of considerations. Existing, commercially available turbomolecular pumps generally fall in the category of high capacity pumps having capabilities usually in the range of 150 to 650 liters/sec. of air. As may be appreciated, such units are comparatively large and complex devices and designed for pumping down large vacuum chambers and are adapted to be run over long operating cycles.
The nature of the turbomolecular pump is such that its effectiveness is quite dependent upon the ambient pressure to which it exhausts. Commonly, restrictions of an exhaust forepressure of 10.sup..sup.-2 to 10.sup..sup.-3 Torr are specified for the pump to reach its designed high vacuum capability. It should be immediately recognized that the specified low exhaust pressure thus requires a substantial forepumping by an auxiliary device. It is usual that oil-sealed rotating around pumps having a capacity of 100 to 200 liters are specified as forepumps adequate for turbomolecular installations.
There presently does not exist in the industry a relatively low cost, low volume, high vacuum pumping system adequate for intermittent duty cycling such as in scientific instrument applications. It is with the foregoing in mind that the present pump was invented.
In scientific instruments involving corpuscular beams, it is usual that evacuated chambers wherein these electron or ion beams are generated and directed upon a target, that backstreamed hydrocarbons can cause serious contamination within the chamber. Further, it is usual that the evacuated chamber is well sealed and of limited volume such that high capacity pumps are not required. However, it is also usual that the degree of evacuation required in many such scientific instruments is very high (eg. 10.sup..sup.-9 Torr in the gun region of a field emission electron microscope). Thus, it must be recognized that a vacuum pumping system for such an instrument must be capable of producing high vacuum, while not necessarily being of great quantitative pumping capacity.
Further, in the case of a vacuum system suitable for a scientific instrument, the pump must be capable of reaching full operating characteristics in a relatively short time and over an often repeated duty cycle.
Thus, while the ultra-high vacuum capacity of turbomolecular pumps would seem to offer advantages to such as scientific instrument applications, their vast size and expense, as well as their dependence upon forepumps has led the industry to seek other alternatives, such as ion pumping and similar devices and to turn away from turbomolecular pumping. It was not until the present developments wherein the principles of turbomolecular pumps were combined with the characteristics of other pumping systems that an integral instrument of versatility and operability was provided to the scientific instrument industry.